Recently there has been a dramatic increase in the need to join fine circuits, for example, a liquid crystal display (LCD) and a tape carrier package (TCP), TCP and a printed circuit board (PCB), and the like. An anisotropic conductive adhesive in which conductive particles are dispersed in a resin is used for such purposes. The anisotropic conductive adhesive is placed between parts which are to be connected to each other, and is heated and is pressed. Adjacent terminals in a direction parallel to the substrate surface are still insulated, but adjacent terminals in a direction perpendicular to the substrate surface are conductive. The reason why an anisotropic conductive adhesive has been commonly used to connect materials as explained above is because conventional methods, for example, soldering and the like, cannot be applied to the devices. Materials for the devices explained above are normally not heat resistant and if the conventional methods are applied to the devices there is a risk of a short-circuit between adjacent terminals in the fine circuits.
There are two types of anisotropic conductive adhesives, i.e., a thermoplastic resin type and a thermosetting resin type. The thermosetting resin type is especially an epoxy resin which is considered more reliable and is becoming more widely used than the thermoplastic resin type because of problems of the thermoplastic resin type as explained below.
A styrene copolymer, for example, styrene-butadiene-styrene (SBS), styrene-isoprene-styrene(SIS), styrene-ethylene-butadiene-styrene(SEBS) and the like, are mainly used as the thermoplastic resin type of anisotropic conductive adhesive. The thermoplastic resin type adhesive is fusible, and it is believed that its processability is better than the thermosetting resin type adhesive because it requires a relatively low temperature and short time for bonding. However, the thermoplastic resin type is not practically acceptable since it cannot give satisfactory results in a long term environmental test because of insufficiency of bonding reliability caused by low water-vapor resistance and chemical resistance.
On the other hand, a thermoset epoxy resin is recently mainly used as the thermosetting resin type anisotropic conductive adhesive because the thermoset epoxy resin has generally well-balanced storage stability and curing characteristics. It is necessary, however, to heat and cure the epoxy resin at a temperature of 150.about.200.degree. C. and for about 30 seconds from a practical standpoint. It is difficult to cure within a practical cure time at a temperature of less than 150.degree. C.
It has been proposed to provide an adhesive containing a latent curing agent, for example, BF.sub.3 amine complex, dicyandiamide, a hydrazide of an organic acid, an imidazole compound and the like, to improve storage stability of a thermosetting resin type anisotropic conductive adhesive. If the adhesive has improved storage stability, however, it takes a longer time or a high temperature to cure. On the other hand, if the adhesive can cure in a shorter time at a low temperature, it does not have a good storage property. Both are unsatisfactory.
Although when a thermoplastic resin type anisotropic conductive adhesive is used, it is not difficult to repair defects after bonding because the resin, which is not cured, is easy to dissolve with a solvent, it is difficult to debond fine circuits after they have been bonded with a thermoset resin type anisotropic conductive adhesive to correct defects, for example, misalignment, without breakage or damage of the bonded parts. Such requirement conflicts with the properties of the thermoset resin type adhesive, i.e., high-adhesive properties and good bonding reliability. None of the conventional thermoset resin type adhesives can satisfy this requirement.
There has rapidly been developed a LCD module having a big screen and narrow frame and being extremely precise. It has been accompanied by miniaturizing of bonding pitch or narrowing of bonding width. When LCD and TCP are bonded, misalignment in the bonding pattern occurs because of stretching of TCP, or internal components of LCD are damaged by high bonding temperature because a bonding portion is very narrow. After LCD is bonded with TCP, and then TCP is bonded with PCB, PCB and LCD are warped because PCB tends to be stretched by heat at bonding, but heat is not transferred to LCD so much and LCD has a small linear expansion. Therefore, breaking of TCP wiring occurs.
Bonding at a low temperature has been attempted in order to solve the problems explained above. Although it is possible to bond by a conventional thermoplastic resin type anisotropic conductive adhesive at a low temperature, bonding reliability is low because water vapor resistance and heat resistance of the resin are low. On the other hand, when an epoxy resin, which is the major thermoset resin for an anisotropic conductive adhesive is used, it is necessary to extend bonding time to cure the resin. This is not practical.
It has also been proposed that conductive particles are dispersed in a resin comprising a cationic polymerization material and sulfonium salt as an anisotropic conductive adhesive capable of bonding at low temperature (Japanese Patent Laid-Open Publication No. Hei 7-90237). It has been further proposed to disperse conductive particles in a composition of an epoxy resin and a 4-(dialkyl amino)pyridine derivative (Japanese Patent Laid-Open Publication No. Hei 4-189883). However, these proposals are not satisfactory for practical application because of problems, for example, insufficient storage stability of the adhesive resin, corrosion of bonded circuit terminals and the like.
An anisotropic conductive adhesive has been proposed which can be cured at low temperatures in which conductive particles are dispersed in a resin composition containing a radical polymerization resin, an organic peroxide, a thermoplastic elastomer, and a maleimide in which the radical polymerization resin is a methacryloyl phenolic novolak resin. An adhesive in which an amino silane coupling agent is added to improve adhesive properties and bonding reliability has also been proposed. However, an adhesive resin, which has well-balanced properties, i.e., curing characteristics, processability, adhesive properties at high temperature and high humidity, bonding reliability, storage properties and the like, has not been obtained. There is a strong demand for an anisotropic conductive adhesive that can cure in a short time and has excellent adhesive properties, bonding reliability, storage stability, repair properties and the like.